Genome engineering includes altering the genome by deleting, inserting, mutating, or substituting specific nucleic acid sequences. The alteration can be gene or location specific. Genome engineering can use nucleases to cut DNA, thereby generating a site for alteration. In certain cases, the cleavage can introduce double-stranded breaks in the target DNA. Double-stranded breaks can be repaired, e.g., by endogenous non-homologous end joining (NHEJ) or homology-directed repair (HDR). HDR relies on the presence of a template for repair. In some examples of genome engineering, a donor polynucleotide, or portion thereof, can be inserted into the break.
Clustered regularly interspaced short palindromic repeats (CRISPR) and associated Cas proteins constitute the CRISPR-Cas system. This system provides adaptive immunity against foreign DNA in bacteria (Barrangou, R., et al., “CRISPR provides acquired resistance against viruses in prokaryotes,” Science 315, 1709-1712 (2007); Makarova, K. S., et al., “Evolution and classification of the CRISPR-Cas systems,” Nat Rev Microbiol 9, 467-477 (2011); Garneau, J. E., et al., “The CRISPR/Cas bacterial immune system cleaves bacteriophage and plasmid DNA,” Nature 468, 67-71 (2010); Sapranauskas, R., et al., “The Streptococcus thermophilus CRISPR/Cas system provides immunity in Escherichia coli,” Nucleic Acids Res 39, 9275-9282 (2011)). The RNA-guided Cas9 endonuclease specifically targets and cleaves DNA in a sequence-dependent manner (Gasiunas, G., et al., “Cas9-crRNA ribonucleoprotein complex mediates specific DNA cleavage for adaptive immunity in bacteria,” Proc Natl Acad Sci USA 109, E2579-E2586 (2012); Jinek, M., et al., “A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity,” Science 337, 816-821 (2012); Sternberg, S. H., et al., “DNA interrogation by the CRISPR RNA-guided endonuclease Cas9,” Nature 507, 62 (2014); Deltcheva, E., et al., “CRISPR RNA maturation by trans-encoded small RNA and host factor RNase III,” Nature 471, 602-607 (2011)), and has been widely used for programmable genome editing in a variety of organisms and model systems (Cong, L., et al., “Multiplex genome engineering using CRISPR/Cas systems,” Science 339, 819-823 (2013); Jiang, W., et al., “RNA-guided editing of bacterial genomes using CRISPR-Cas systems,” Nat. Biotechnol. 31, 233-239 (2013); Sander, J. D. & Joung, J. K., “CRISPR-Cas systems for editing, regulating and targeting genomes,” Nature Biotechnol. 32, 347-355. (2014)).
Jinek, M., et al., (“A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity,” Science 337(6096):816-21 (2012)) showed that in a subset of CRISPR-associated (Cas) systems the mature CRISPR (crRNA) that is base paired to trans-activating crRNA (tracrRNA) forms a two-part RNA structure that directs the CRISPR-associated protein Cas9 to introduce double-stranded breaks in target DNA. At sites complementary to the crRNA-guide (spacer) sequence, the Cas9 HNH nuclease domain cleaves the complementary strand and the Cas9 RuvC-like domain cleaves the non-complementary strand. Dual crRNA/tracrRNA molecules were engineered into single-chain crRNA/tracrRNA molecules. These single-chain crRNA/tracrRNA directed target sequence-specific Cas9 double-strand DNA cleavage.
Jinek, M., et al., designed two versions of single-chain crRNA/tracrRNA containing a target recognition sequence (spacer) at the 5′ end followed by a hairpin structure retaining the base-pairing interactions that normally occur between the tracrRNA and the crRNA (see FIG. 5B of Jinek, M., et al.). For each single-chain crRNA/tracrRNA, the 3′ end of crRNA was covalently attached to the 5′ end of tracrRNA. In cleavage assays using plasmid DNA, Jinek, M., et al., observed that a 3′ truncated single-chain crRNA/tracrRNA did not cleave target DNA as efficiently in the assay as a longer single-chain crRNA/tracrRNA that was not truncated at the 3′ end (see FIG. 5B and FIGS. 14 A, B, and C of Jinek, M., et al.). These data confirmed that the “5 to 12 positions beyond the tracrRNA:crRNA base-pairing interaction are important for efficient Cas9 binding and/or target recognition” (Jinek, M., et al., Science 337(6096):820 (2012)).
Briner, A., et al., (“Guide RNA Functional Modules Direct Cas9 Activity and Orthogonality,” Molecular Cell 56(2), 2014, Pages 333-339) elucidated the molecular basis of selective Cas9/guide-RNA interactions by identifying and characterizing distinct sequence and structural modules within guide RNAs that direct Cas9 endonuclease activity and define orthogonality. They established six modules within native crRNA:tracrRNA duplexes and single guide RNAs (sgRNAs) across forty-one systems from three distinct Cas9 families. The six identified modules are the spacer, the lower stem, the bulge, the upper stem, the nexus hairpin, and 3′ hairpins. These modules are illustrated with reference to an sgRNA in FIG. 2.
Using the sgRNA/Cas9 system from Streptococcus pyogenes, Briner, A., et al., showed that a bulge within the sgRNA is structurally necessary for DNA cleavage both in vitro and in vivo, whereas sequence substitutions are tolerated in other regions. Furthermore, expendable features can be removed to generate functional miniature sgRNAs. They also identified a conserved module “named the nexus; this feature exhibits sequence and structural features important for cleavage” (Briner, A., et al., page 2). They stated that this module, the nexus, is “necessary for DNA cleavage” (Briner, A., et al., Summary). The nexus hairpin confers activity to its cognate Cas9. The location of this nexus hairpin corresponds to the 5 to 12 positions beyond the tracrRNA:crRNA base-pairing interaction that are important for efficient Cas9 binding and/or target recognition as identified by Jinek, M., et al. (see above).
Briner, A., et al., showed that the general nexus hairpin shape with a GC-rich stem and an offset uracil was shared between the two Streptococcus families. In contrast, the idiosyncratic double stem of the nexus hairpin was unique to, and ubiquitous in, Lactobacillus systems. Some bases within the nexus hairpin were strictly conserved even between distinct families, including A52 and C55, further highlighting the important role of this module. In the crystal structure of SpyCas9 A52 interacts with the backbone of residues 1103-1107 close to the 5′ end of the target strand in the in the crystal structure of SpyCas9, suggesting that the interaction of the nexus hairpin with the protein backbone may be required for protospacer-adjacent motif (PAM) binding.
Wright, A. V., et al., (“Rational design of a split-Cas9 enzyme complex,” PNAS 112(10), 2015, pages 2984-2989) determined the RNA molecular determinants of sgRNA motifs that promote heterodimerization of the α-helical and nuclease lobes to form a ternary complex. Crystal structures of sgRNA/DNA-bound Cas9 showed that the spacer and the stem-loop motifs (i.e., the lower stem, the bulge, and the upper stem modules described by Briner, A., et al.) at the 5′ end of the sgRNA primarily contact the α-helical lobe, whereas the two hairpins (i.e., the hairpins module described by Briner, A., et al.) at the 3′ end bind the outside face of the nuclease lobe. They noted that “the nexus motif, recently shown to be critical for activity” (Wright, A. V., et al., page 2986, col. 1), occupies a central position between the lobes and forms extensive interactions with the bridge helix. Based on this interaction profile, Wright, et al., generated a full-length sgRNA and two shorter sgRNA constructs that were selectively truncated from either the 5′ or 3′ end (no modifications were made to the critical nexus hairpin) and determined their affinities for wild-type Cas9, the individual α-helical and nuclease lobes, and split-Cas9.
Contrary to the above-described teachings of the prior art, experiments performed in support of the present invention unexpectedly demonstrated that Cas9 functions (e.g., binding and cutting double-strand DNA) are supported by guide RNAs having a split nexus, as well as guide RNAs having modifications of the split nexus.
Results presented in the present specification open new design and engineering avenues for CRISPR technologies and set the stage for the development of next-generation CRISPR-based technologies.